Voiceband modems for providing digital communications between computers via twisted pair telephone lines are well known. Voiceband modems are commonly used to provide Internet access by facilitating digital communications between personal computers and Internet Service Providers (ISPs).
Because voiceband modems operate within the limited bandwidth of the Public Switched Telephone Network (PSTN), i.e., 0 Hz to 3,400 Hz, they are only capable of providing data rates up to approximately 56 Kbps.
Due to the increasingly large quantity of digital data being communication via twisted pair telephone lines, the maximum bit rate associated with voiceband modems is frequently considered inadequate. The comparatively slow speed of voiceband modems is a severe limitation when transferring large binary files such as images, film clips, audio, large data files and the like. At 56 Kbps, such files may require an undesirably long amount of time to transfer between computers. Further, many applications, such as those involving realtime video, are not possible at 56 Kbps.
In an attempt to mitigate the data transfer rate limitation associated with such contemporary voiceband modems, integrated services data network (ISDN) modems have been developed. Not only do such ISDN modems increase the data rate to approximately 112 Kbps in some instances, but ISDN also facilitates the use of multiple communications devices simultaneously. For example, an ISDN system may be configured so as to allow the simultaneous transmission of data from a computer and voice from a digital telephone. The use of ISDN necessitates the installation of an ISDN compatible switch by the telephone company.
The increased data rate of ISDN modems with respect to voiceband modems is due both to the use of a substantially larger frequency spectrum, i.e., 0 Hz to 80 kHz instead of 0 Hz to 3,400 Hz, and to the use of a more advanced coding technique, i.e., 2B1Q. According to 2B1Q coding, two bits are transmitted with each symbol, thereby doubling the bit rate.
The increasing popularity of such communication services as video on demand (pay-per-view), realtime video teleconferencing and high speed Internet access has further increased the need for higher data rates over twisted pair telephone lines. Even the comparatively high speed associated with ISDN is not adequate for providing such services, which typically require data rates of at least 1.5 Gbps.
Digital subscriber line (DSL) provides a way of facilitating digital communications over twisted pair telephone lines at data rates in excess of 1.5 Mbps, so as to facilitate such desirable services as video on demand, realtime video teleconferencing, high speed Internet access and the like.
It is worthwhile to note that although fiber optic cable will provide data rates in excess of those possible utilizing DSL on twisted pair telephone lines, the installation of fiber optic cable to customer premises is costly and is expected to take more than a decade. Therefore, it is necessary to leverage (such as via DSL) existing twisted pair copper wiring. It should be noted that this alternative is particularly attractive to telephone companies, since their existing infrastructure provides the telephone companies with a distinct time-to-market advantage in the highly competitive communications business.
There are currently several different versions of DSL available. These include basic digital subscriber line (DSL), high data rate digital subscriber line (HDSL), single line digital subscriber line (SDSL), asymmetric digital subscriber line (ADSL) and very high bit rate digital subscriber line (VDSL).
Basic DSL provides a data rate of 160 Kbps simultaneously in both directions over a single twisted pair of telephone lines for distances of up to approximately 18,000 feet.
HDSL is an extension of basic DSL and provides an improved method for transmitting T1/E1 signals. T1 is used primarily in North America and Japan and facilitates the simultaneous transmission of 24 digitized voice channels. E1 is used in most of the rest of the world and supports up to 30 simultaneous digitized voice channels.
HDSL uses an advanced modulation technique to facilitate a data rate of 1.544 Mbps over a twisted pair telephone line for a distance of up to approximately 12,000 feet. HDSL requires two twisted pair telephone lines, each twisted pair operating at 768 Kbps.
SDSL is a single line version of HDSL. In SDSL, T1/E1 signals are communicated over a single twisted pair. SDSL is suitable for such applications as servers and power LANs, which require symmetric data communications, wherein equal data rates in both the upstream and downstream directions are provided. SDSL is also suitable for such services as private line and frame relay.
ADSL is well suited for video on demand, home shopping, Internet access and remote LAN access, wherein the downstream data rate is comparatively high with respect to the upstream data rate. As mentioned above, the communication of video, such as MPEG movies, can require data rates in excess of 1.5 Mbps. However, this high bit rate is in the downstream direction only. The upstream control signals, which may be from simulated VCR controls, typically require as little as 16 Kbps. It has been found that a ten to one ratio of downstream to upstream data rates is suitable for many such data communications applications.
VDSL, like ADSL, utilizes asymmetric data communications. However, VDSL operates at much higher data rates, which are facilitated by requiring shorter transmission distances via the twisted pair telephone lines. Further, a symmetric version of VDSL may be utilized in multimedia applications requiring similar data rates in both directions.
Approximately 700 million twisted pair copper telephone lines presently interconnect homes and businesses worldwide. Because of this large installed base, telephone companies have a distinct advantage over cable companies in the marketing of data communications services. Further, the cable companies use a shared transmission medium system, wherein a single coaxial cable services a plurality of computers. Thus, although a cable modem may, in some instances, provide higher data rates due to the use of the high bandwidth coaxial cable transmission medium rather than twisted pair copper telephone lines, the data rate actually achieved by a cable modem depends to a great extent upon the number of computers sharing the coaxial cable simultaneously. As more computers communicate via the same coaxial cable, each individual computer's data rate is reduced proportionally.
This reduction in data rate occurs as the plurality of individual computers compete for the limited transmission medium bandwidth of the shared coaxial cable. When more computers simultaneously communicate via the coaxial cable, an inherently smaller bandwidth allocation is provided to each individual computer.
However, when computers communicate via dedicated telephone lines, by way of contrast, their data rate is independent of the communication activities of other computers. Thus, the use of telephone lines to facilitate digital communications has a distinct advantage over cable modem systems. Consequently, the larger installed base of twisted pair telephone lines, as compared to coaxial cable, combined with the dedicated communications capability provided by such telephone lines, makes twisted pair communications a viable alternative to cable modem technology.
The various different types of DSL may be referred to collectively as either DSL or XDSL. DSL utilizes an advanced modulation scheme known as quadrature amplitude modulation (QAM), wherein a combination of amplitude and phase modulation is used to encode digital information for transmission over twisted pair copper telephone lines. QAM is an extension of multiphase shift keying modulation schemes, such as quadrature phase shift keying (QPSK). The primary difference between QAM and QPSK is the lack of a constant envelope in QAM versus the presence of a constant envelope in phase-shift keying techniques.
QAM is based upon suppressed carrier amplitude modulation of two quadrature carriers, i.e., two carriers having a phase relationship of 90 degrees with respect to one another.
Although QAM can have any number of discrete digital levels which the physical media will accommodate, common levels are QAM-4, QAM-16, QAM-64 and QAM-256, wherein the number indicates how many discrete digital levels are utilized.
Thus, it will be appreciated that the use of QAM facilitates the simultaneous transmission of a larger number of bits, e.g., up to 256 bits with QAM-256, so as to provide substantially enhanced bit rates. Each such simultaneous transmission of a plurality of bits is accomplished by encoding the bits into a symbol. Of course, the use of symbols which contain a larger number of bits requires higher signal to noise ratios(SNR).
Although QAM does provide a substantial increase in bit rate, as compared with earlier modulation schemes such as those which are utilized in contemporary voiceband modems and ISDN modems, it is still desirable to optimize the bit rate provided by QAM, so as to provide digital communications at the highest possible speed while maintaining the desired quality of service.
One major problem which inhibits optimization of the bit rate in DSL installations is radio frequency ingress (RFI). RFI occurs when the twisted pair copper wires of a DSL installation function as a radio antenna at the frequencies upon which the DSL transceivers communicate. Although the receiver front ends of DSL transceivers include differential amplifiers and such RFI is generally coupled to the twisted pair in the common mode, not all of the undesirable RFI is eliminated by the differential amplifiers. Leakage of some portion of the RFI past the differential amplifiers inherently occurs since it is not possible to define a twisted pair transmission medium which is completely balanced. Thus, some portion of the RFI is induced in a differential mode and/or some portion of the common mode induced RFI is converted to a differential mode. Of course, any portion of the RFI which is within the frequency range of the differential amplifiers and which is in the differential mode at the input to the differential amplifiers is processed by the differential amplifiers in the same manner as the desired received signal, i.e., is amplified and passed on for further processing. Thus, such differential mode induced RFI undesirably interferes with the received signal and thereby degrades the performance of the DSL system, resulting in an undesirably reduced bit rate.
Therefore, it is desirable to provide a method and apparatus for mitigating the effects of such undesirable RFI, so as to enhance the bit rate in a DSL communications system or the like.
Another problem associated with DSL installations which inhibits optimization of bit rate is intersymbol interference (ISI). ISI occurs in communication systems as the symbols being communicated over a physical medium tend to spread out in time, so as to overlap and substantially interfere with one another. ISI which occurs as a result of one symbol spreading backwardly into another, subsequent, symbol is known as post-cursor ISI. Similarly, ISI caused by one symbol spreading forwardly so as to interfere with another, preceding, symbol is known as pre-cursor ISI. As those skilled in the art will appreciate, when the post-cursor ISI of one symbol, for example, extends into a subsequent symbol, this post-cursor ISI increases the amplitude of the subsequent symbol, thereby potentially causing the subsequent to be misinterpreted by the slicer of the receiver. Of course, pre-cursor ISI has the same detrimental effect on a preceding symbol.
ISI becomes much more detrimental as bit rates increase. As bit rates increase, symbols become much more tightly packed in time, i.e., become closer to one another, such that the symbols are much more subject to the effects of pre-cursor and post-cursor ISI. That is, since the symbols are closer to one another, each symbol overlaps a greater (higher amplitude) portion of the spread portion of an adjacent symbol. Because ISI undesirably interferes with the proper interpretation of the symbols by the slicer, the bit rate must generally be reduced to the point where ISI is acceptable, i.e., does not result in an excessive bit error rate (BER). Therefore, ISI tends to substantially inhibit optimization of bit rate. Thus, it would additionally be desirable to provide a method and apparatus for mitigating the undesirable effects of ISI, so as to facilitate the optimization of bit rate in DSL communications systems and the like.